All Around The Globe: USGS Raises Alert Level For World’s Most Active Volcano

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While it’s not imminent, observers at the U.S. Geological Survey say the world’s largest active volcano is nearing a new period of eruption.

Mauna Loa on Hawaii’s Big Island last erupted in 1984, but the USGS has always considered it an active volcano. It adjusted its alert level from “normal” to “advisory” after a steady increase in the number earthquakes felt in the area—now reaching up to 75 per week—that began in March.

The USGS status report for Mauna Loa, filed on Tuesday, states:

“As has happened before, it is possible that current low-level unrest will continue and vary in intensity for many months, or even years without an eruption. It is also possible that the current unrest is an early precursor to an eventual eruption. At this time, we cannot determine which of these possibilities is more likely.

“HVO expects that days or weeks prior to an eruption, monitoring instruments will detect signs of an increased potential for eruption. These signs could include further increases in rates of earthquakes and ground deformation, increases in the sizes of earthquakes, an increase in surface temperatures, or an increase in visible steam plumes or sulfur dioxide emissions.

“However, it is also possible that the timeframe to eruption could be shorter – hours to days. All communities on the flanks of the volcano should be prepared.

“HVO continues to monitor the volcano closely and will report any significant changes. HVO is in close touch with Hawaii County Civil Defense and other agencies responsible for public safety.”

While the “advisory” status is the next-to-lowest for an active volcano, Mauna Loa has a history of rapidly “evolving” after periods of inactivity. Once an eruption occurs, lava typically flows from the volcano rapidly, reaching the ocean “on the order of a couple hours.”

The volcano has erupted 33 times since 1843. Last time, the lava flows ended less than five miles from Hilo, which is the largest city on the Big Island, in what was one of its largest eruptions ever recorded.